In the last few years, the substantial success of CMOS technology has been determined basically by the possibility of constantly reducing the size of electronic devices. In fact, this technology follows the so-called Moore's law, according to which the number of transistors that can be made on an integrated circuit and hence the computing speed should double in a time range of between 18 and 24 months.
However, it is a common conviction that conventional silicon microelectronics will not be able to continue indefinitely to follow this law, in so far as sooner or later physical limits will certainly be reached, which will prevent current circuits from functioning reliably at nanometric dimensions, whereas, at the same time, an exponential increase in production costs will render any further increase in the levels of integration prohibitive. In fact, as the density of the electronic devices on a chip increases, phenomena such as the need to dissipate the heat generated by such densely populated circuits, and the transition from the classical behaviour to the quantistic behaviour of the charge carriers will considerably slow down progress.
In particular, thanks to the use of lithographic techniques, sizes of the order of 100 nm have currently been reached. Notwithstanding the rapid progress achieved in the current process of scale integration, present-day technology is unlikely to allow further scaling beyond this critical size. In fact, once this critical size has been reached, the small electric currents which carry the information transfer uncontrollably vary from one device to another. In particular, when the quantistic effects start to be important, transistors tend to lose the electrons that represent the information, so that it becomes difficult to keep them in their original state. It is foreseeable that, below the size referred to above of 100 nm, the said difficulties will become of major importance.
The need to solve the above problems has pushed research towards the study of new technologies based upon the use of organic materials which could replace silicon either completely or in part in the manufacture of electronic devices.
Molecular electronics affords the potential for overcoming the limits of on-silicon technology, in so far as it is possible to manufacture single-molecule devices which are organized in parallel by means of self-assembly techniques that are also economically advantageous.
In particular, whilst it is possible, with current technologies, to produce high-density flash-memory units with capacities in the region of 64-128 Mb, but with high costs, in the future, by resorting to memories of a molecular type, it will be possible to achieve memory capacities of various tens or hundreds of gigabits, in which each memory cell will be made up of one or more molecules, the structural configuration of which may be associated to different possible and modifiable states by means of interactions at an atomic level or at the level of individual electrons.
There has consequently arisen the need to explore the possibility of passing from the current techniques of assembly of a top-down type, whereby it is possible to arrive at the desired dimensions, by successive removal of a macroscopic amount of a material, to bottom-up technologies, whereby it is possible to manufacture, and subsequently assemble, nanoscopic components, starting from individual atoms or molecules, i.e., components in which the devices involved in management and retention of data are molecules arranged and interconnected so as to form a circuit.
The range of prototypes of molecular devices proposed is very wide, as emerges from the review article by C. Joachim, J. K. Gimzewski, A. Aviram, Nature 408, 541 (2000).
In the above prototypes, the problem of monitoring the molecule, so as to bring its behaviour back to the macroscopic world, has up to now been approached with the introduction of the scanning-tunnelling microscope (STM).
Nonetheless, up to now the different proposed solutions do not enable large-scale integration of molecular electronics for industrialization purposes.
Amongst the different prototypes of molecular memories which have been proposed and which use different types of molecules, two are worthy of particular attention: the nonvolatile memory based upon the use of the bipyridine molecule manufactured at the University of Liverpool and described in Gittins et al., Nature 408, 67 (2000), and the molecular memory based upon the use of the rotaxane molecule, described in the U.S. Pat. No. 6,128,214, granted on October, 2000, in the name of Hewlett-Packard.
In particular, in a molecular memory, retention of the information bit is entrusted to a molecule which acts as a switch. In detail, the University of Liverpool has made a switch that is also based on the change of the chemical state of oxidation of a molecule. In practice, a monolayer has been made, containing molecules with a bipyridinium group, attached to an electrode and to a nanocluster, both made of gold, through thiol chains arranged at the two ends, as illustrated in FIG. 1. The tip of a scanning-tunnelling microscope detects the flow of current through the molecular switch. The electrons can be injected into the bipyridinium group by applying an appropriate potential difference between the substrate and an electrode arranged alongside the self-assembled monolayer, immersed in the surrounding electrolyte. When both of the nitrogen atoms on the benzene cycles are oxidized, no current flows. Instead, when an electron is added on one of the two nitrogen atoms, a big current flows. The size of this device is 10 nm.
Instead, in the U.S. Pat. No. 6,128,214, the switching function is carried out by the rotaxane molecule, a synthetic molecule which has two states, only one of which enables passage of current. Broadly speaking, the molecule, arranged between two conductive wires, functions as a bridge between the wires: when the molecule is present, the electrons can flow from one wire to the other, and the switch closes the circuit. If an electric field is applied, the rotaxane changes configuration, the bridge disappears and the switch opens the circuit. In particular, the molecule behaves as a switch that closes and conducts by resonant tunnelling when it is in the reduced state, whilst it behaves as a switch that opens and hence functions as a barrier for tunnelling conduction when it is in its oxidized state.
In greater detail, the storage device described in the U.S. Pat. No. 6,128,214 is illustrated schematically in FIGS. 2a and 2b and is made up of two parallel layers of nanometric conductors arranged perpendicular to one another, between which there is interposed a layer of rotaxane molecules. A bistable rotaxane molecule is arranged in a position corresponding to each intersection of the pairs of conductors and in effect defines a switch. An applied voltage modifies the electronic state of the molecules and, hence, the resistance between the top conductor and the bottom conductor. The switches are activated by electrochemical oxidation or reduction of the molecules. When a switch is electrochemically closed, the resistance between the connected conductors is low, and this state constitutes a logic “1”. When the switch is open, the resistance is high, and this state represents a logic “0”. In order to read the state of the switch, it is necessary to apply another voltage, the value of which is such as not to cause switching of the state of the junction. Consequently, the reading process is not destructive.
With the molecules described above, it is thus possible to make memory cells using connectors between the molecules as crossbar system, which enable writing and reading of the state of each individual memory element.
The limits of the above molecular memories are basically linked to the poor efficiency of the contacts, i.e., the difficulty of arranging the molecules in contact with the nanometric conductors, which are made of silicon or metal, to the instability of the molecules at high temperatures, and to the possibility of giving rise to multiple current paths.
In order to overcome the drawbacks of the molecular memories described above, in the article “Carbon Nanotube-Based Nonvolatile Random Access Memory for Molecular Computing”, Science, Vol. 289 (5476), Jul. 7, 2000, 94-97, there is proposed the manufacture of nonvolatile memories based upon the use of molecular structures known as carbon nanotubes (CNTs).
It is known that carbon atoms have the property of organizing themselves to form different structures, giving rise to materials of different forms. In fact, a diamond is made up of carbon atoms organized in tetrahedrons, whilst graphite is made up of carbon atoms organized in planar structures. These two allotropic forms, albeit originating from the same type of atoms, exhibit structural properties (hardness, elasticity, friction) and functional properties (electrical conductivity, colour, etc.) that are highly different and frequently opposite to one another. The structural characteristics, such as hardness and refractoriness, of graphite and diamond render a top-down approach hard to implement on a device at nanometric scales. Instead, a bottom-up approach is rendered possible by the use of another allotropic form of carbon, namely fullerene.
Belonging to the family of the fullerenes is C60, also known as buckyball, which presents a molecular structure having the shape of a polyhedral cage, consisting of pentagons and hexagons. The fullerene structures that develop as long cylinders, rather than as spheres, are called nanotubes. Their length (of several microns) may be thousands of times greater than their diameter (of the order of nanometers). In addition, using known techniques of molecular synthesis, there have been observed, in the laboratory, single-walled cylindrical structures (single walled nanotubes—SWNTs), having a diameter of 1-2 nm, and multiple-walled structures (multiple walled nanotubes—MWNTs), i.e., made up of coaxial cylinders with diameters of a few tens of nanometers.
Carbon nanotubes are organic molecules made up of a number of interconnected carbon atoms in a cylindrical structure, characterized by a small weight, and they present exceptional elastic properties, which render them extremely hard, but also capable of undergoing large deformations without breaking. Thanks to their exceptional property of conducting electrical charges, carbon nanotubes, since they can be configured both as conductors and as semiconductors, are suitable for forming components of a new class of nanometric electronic devices. In particular, they are expected to play a primary role in the development of molecular electronics, owing to the fact that, thanks to their lateral dimensions (of the order of nanometers) and to their electrically conductive properties, they behave as quantum conductive wires of nanometric dimensions (the so-called quantum nanowires).
Carbon nanotubes have different shapes, which can be described by a vector, referred to as chiral vector C, as illustrated in FIG. 3.
In particular, in geometrical terms, a carbon nanotube (CNT) can be obtained from a sheet of graphite, by “cleaving” it along the lines (represented by dashed lines in FIG. 3) perpendicular to the chiral vector, and then “rolling” it up in the direction of the chiral vector. In this way, there is formed a cylinder of diameter d=|C|/π.
The chiral vector C can be set in relation to two unit vectors, designated by a1 and a2, which define the lattice of the planes in the graphite, by means of two indices n and m, according to the following equation:C=n·a1+m·a2.
Linked to the above indices n and m are an angle φ, referred to as chiral angle, and the diameter d of the nanotube, according to the following equations:
                    ϕ        =                  arccos          ⁢                      ⌊                                          3                            ⁢                                                (                                      n                    +                    m                                    )                                /                2                            ⁢                                                (                                                            n                      2                                        +                                          m                      2                                        +                                          n                      ⁢                                                                                          ⁢                      m                                                        )                                                      ⌋                                                  d        =                              a            π                    ⁢                                                    n                2                            +                              m                2                            +                              n                ⁢                                                                  ⁢                m                                                        
The values of the indices n and m define the chirality of the nanotube, which is the state of the nanotube and which is different according to the way in which the hexagons of the graphite are arranged to form the cylindrical structure. The chirality of a nanotube is, therefore, given by the pair of integer indices (n, m), and determines the structural characteristics and hence the conductive properties of a nanotube. In particular, as regards the structure, nanotubes that have the indices n and m equal, i.e., nanotubes (n, n) are referred to as “armchair” nanotubes, on account of the arrangement of the hexagons of graphite with respect to the axis of the nanotube itself. Nanotubes in which one of the two indices is zero (n, 0), are referred to as “zigzag” nanotubes, whereas nanotubes with different indices are referred to, in general, as chiral nanotubes. The chirality conditions the conductance of the nanotube, its density, the reticular structure, and other properties. The chiral indices may, in principle, be obtained experimentally by measuring the chiral angle φ and the diameter d of the nanotube with a transmission-electron microscope (TEM) or with a scanning-tunnelling microscope (STM).
In addition, according to their chirality, nanotubes may be semiconductor nanotubes or metallic nanotubes. In fact, nanotubes where the indices of chirality of which satisfy the following relation:n−m=3·l =0, 1, 2,are metallic and hence conductors. All the others present a non-zero bandgap and, consequently, behave as semiconductors. Armchair nanotubes are metallic.
The fundamental bandgap of a semiconductor carbon nanotube depends upon the diameter d of the nanotube, according to the following relation:Egap=2y0acc/dwhere y0 is the binding energy of the carbon atoms, and acc is the distance between two neighbouring carbon atoms.
Consequently, by appropriately modifying the chirality of the nanotube and hence its diameter, it is possible to modulate its bandgap. The two different geometrical structures of the molecule (i.e., the initial one and the modified one) can thus represent two stable states.
Carbon nanotubes can be produced in macroscopic amounts using various techniques: laser ablation, arc discharge, or else chemical vapour deposition. For a more detailed treatment regarding this latter technique see, for example, H. M. Cheng et al., Appl. Phys. Lett. 72, 3282 (1998).
In particular, this latter technique is compatible with the methods used in the microelectronics industry and enables nanotubes to be grown on the substrate. Using the various techniques, it has been found that the nanotube that can be produced in the largest quantities is the (10, 10) nanotube.
As has been said, carbon nanotubes represent a solution for meeting the need to reduce the size of devices present in integrated circuits. In fact, by means of these versatile molecules the road has been opened to the construction of molecular transistors. In particular, the first field effect transistor (FET), in which the channel was a nanotube functioning at room temperature, was manufactured by the researchers of Dekker's group at the University of Delft in 1998. For a more detailed treatment of the subject, see, for example, S. J. Tans et al., Nature, 393, 49 (1998). The above team of researchers deposited two gold electrodes (which functioned as source and drain) on a substrate of silicon dioxide grown on silicon (gate) and connected these two electrodes to a cylindrical single wall structure (SWNT), which functioned as channel. They measured the current-voltage characteristic of this three terminal device and found that it respected the characteristic of a field effect transistor.
A further nanometric electronic device is described in the aforementioned article “Carbon Nanotube-Based Nonvolatile Random Access Memory for Molecular Computing” and is illustrated in FIGS. 4a and 4b. The nanometric electronic device basically consists of a substrate (for example, doped silicon), on which nanotubes are arranged, on two different levels, orthogonally with respect to one another and vertically separated by a distance of 1-2 nm, in such a way as to cross at a point corresponding to each memory element. Arranged between the conductive substrate and the first level of nanotubes is a dielectric (for example, silicon dioxide, SiO2), with the purpose of insulating the first level of nanotubes from the substrate, which is biased with a reference voltage (ground). Furthermore, the nanotubes of the second level are arranged on top of spacers made of (organic or inorganic) dielectric material, so as to be insulated from the nanotubes of the first level. Instead, no dielectric is interposed at the points where the nanotubes of the second level and the nanotubes of the first level cross.
The nanotubes are then connected to the rest of the circuit, outside the grid, by means of contacts, made, for example, of gold, these contacts being used both during reading and during writing. In particular, writing of the single memory element is performed by imposing an electrostatic action between the substrate and the point of crossing of the two nanotubes, through the dielectric.
The voltage values imposed depend upon the thickness of the dielectric and upon the energy levels required for guaranteeing a change of state of the nanotube (ON/OFF), in such a way that such changes will be reversible. The OFF and ON states for the individual memory element are shown in FIGS. 5a and 5b, respectively. In particular, a voltage value of 4.5 V for the ON state and a value of 20 V for the OFF state have been estimated.
Consequently, reading of the individual memory element would be done electrically, always by means of the electrical contacts arranged on the edge of the grid, by measuring the resistivity associated to the two states ON/OFF.
The main limitations which, at the moment, prevent industrial development of the above approach are outlined in what follows.
A first limitation is represented by the fact that the techniques used for the manufacture of said architecture do not enable control of the nature of the nanotubes used. In particular, there will be a random distribution of metallic nanotubes (M) and semiconductor nanotubes (S). This implies that, in the read step, the values of resistivity measured on different cells that are in the same state, ON or OFF, undergo, even major, fluctuations according to the geometrical configuration (chirality) of the two nanotubes concerned. For instance, the crossings between nanotubes M-M, M-S, S-S are possible, to which different levels of resistivity are associated, corresponding both to the ON state and to the OFF state, even though the levels of resistivity for the two states ON-OFF remain, basically, distinguishable.
A second limitation, which may prevent operation of the nonvolatile memory architecture described herein, is represented by the possibility that, during reading operations, current paths are set up, which are able to falsify the interpretation of the state of an individual cell. This problem is known, and occurs, for example, in optical scanning systems of a matrix type. By way of example, FIG. 6 illustrates a schematic circuit diagram in which acquisition of the state is in effect falsified as a result of the multiple paths of the current. Generally speaking, this problem is solved using rectifying diodes connected in series to each sensing element so as to prevent the diodes from being traversed by reverse currents. In the case in point, this possibility would risk markedly influencing the complexity of the system, i.e., the final storage capacity, unless the rectification functions can be integrated by acting directly on the metal-semiconductor characteristics of the nanotubes. In this way, it would be possible to generate Schottky junctions, already integrated in the cell array.
A third limitation is represented by the fact that the configuration entails the manufacture, arrangement and manipulation of individual nanotubes, appropriately organized and insulated from the rest of the structure. This operation is very difficult, as well as being costly, in so far as the products of the processes of synthesis are, generally speaking, bundles of nanotubes, and the extraction of a single molecule from the bundle would involve additional process steps, together with the manipulation of the molecules.